1. Field of the Invention
This invention relates generally to a tire pressure monitoring system (TPMS) that monitors the pressure and temperature of each tire on a vehicle and, more particularly, to a TPMS that monitors the pressure and temperature of each tire on a vehicle and uses a received signal strength indicator (RSSI) or Doppler shift information available from the received signal to identify which tire is transmitting the signal that is received by a receiver.
2. Discussion of the Related Art
All new vehicles manufactured and sold in the United States are required to have a tire pressure monitoring system (TPMS) that monitors the pressure and temperature of the tires on the vehicle. Typically, the TPMS is an active system that includes its own power source, such as a battery, and a processor for processing data. The TPMS includes a suitable pressure sensor that measures the pressure within each tire and a suitable temperature sensor that measures the temperature within the tire. Sensor signals from the pressure and temperature sensors are sent to the processor in each tire, which conditions and encodes the data and the encoded signals are then modulated onto a carrier wave to be transmitted by an antenna as a TPMS signal. The TPMS signals transmitted from each tire are received by a receiver at a suitable location within the vehicle, where the received data is demodulated, decoded and used to determine whether a warning signal should be given that the tire pressure is too high or too low, or the tire is too hot.
In one known TPMS, sometimes referred to as a low-line system, each tire transmits the TPMS signal including the pressure and temperature information to a single receiver located at a desirable position on the vehicle, where the TPMS signal includes an identification (ID) code that identifies the tire. During manufacture of the vehicle the TPMS algorithm is programmed so that the receiver knows the location of the tire for a particular ID code, i.e., right-front, left-front, right-rear or left rear. If the vehicle owner rotates the tires or replaces one of the tires with the spare tire, the particular tire or tires are now not in their original location, but the receiver will process the TPMS signals thinking that the tires are in their original position. In this situation, if a warning signal is given that a particular tire is under inflated, the vehicle operator may provide air to the wrong tire, possibly over inflating it.
Currently, on these types of systems, it is necessary for the vehicle owner to take the vehicle to a dealer or service center to retrain the TPMS to identify the new location of the tires on the vehicle if the original location of the tires is changed. Particularly, a technician will put the TPMS into a learn mode through a diagnostic interface on the vehicle, and position a tool including a magnet near the valve stem of each tire, which causes the processor in that tire to transmit its ID code to be received by the receiver. While in the learn mode, the technician will cause the processors in each tire to send the TPMS signal in a certain order so that the receiver relearns the location of which tire is associated with which ID code. Currently, with this type of system, there does not exist a technique where the vehicle owner can readily cause the TPMS to relearn the position of the tires if they are changed.